Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Mutual capacitance touch sensor panels can be formed from a matrix of drive and sense lines of a substantially transparent conductive material such as Indium Tin Oxide (ITO), often arranged in rows and columns in horizontal and vertical directions on a substantially transparent substrate. Signals transmitted through the drive lines pass through the static mutual capacitance at the crossover points (sensing pixels) of the drive lines and the sense lines. The static mutual capacitance, and any changes to the static mutual capacitance due to a touch event, can be determined from sense signals that are generated in the sense lines due to the drive signals.
In order to minimize negative visual effects on the LCD image, the ITO forming the touch sensing pixels should not introduce any color shift, moiré, or visual artifacts of any kind. Ideally, the ITO pixel layer should be invisible to the human eye. To achieve this goal, the thickness of the ITO layer must be less than the thickness threshold where negative visual effects such as color shift become apparent. In large touch screens, because of the large distance over which the ITO must span, the electrical resistance of the drive and sense lines can become so large that the resulting resistance and capacitance (RC) time constant reduces the rate at which the touch screen can be scanned. This can lead to slow response to user input and overall poor system performance. A solution to reducing the RC time constant is to use a thicker ITO coating. However, a uniformly thicker ITO would also introduce an undesirable color shift along with some reduction in overall light transmission.